Abstract : The North Atlantic oceanic airspace NAT joins two highly populated continents - North America and Europe. Air traffic in NAT mainly contributes to two major flows: the westbound flow departing from Europe in the morning, and the eastbound flow departing from North America in the evening. Each of these flows consists of more than 500 aircraft daily. Aircraft within NAT experience strong winds caused by the Jet Streams JSs. JSs are fast narrow air currents mainly located in the upper troposphere and generally moving in eastern direction. The JS speed is typically 100 kts nautical miles per hour but can reach 200 kts. Thus, the eastbound flights would try to exploit the JS in order to benefit from strong tailwinds, while the westbound flights would avoid the JS in order to minimize head-wind. As a result, the NAT is highly congested at peak hours. Standard radar-based surveillance is not available for most part of NAT. To perform the Air Traffic Control and assure the en-route aircraft separation, the Organized Track System OTS was established in NAT. The OTS consists of several quasi-parallel tracks each of which represents a sequence of great circles joining successive significant waypoints WPs. In vertical direction each track consists of several flight levels. The longitudinal in-track separation in NAT is assessed in terms of differences in actual and estimated times of arrival at common WPs; and expressed in clock minutes. Current regulations impose large separation minima for aircraft on OTS.